Instead of a NOS kit, a turbo is the other option to gain more horsepower.
The turbo used is a K04 from Borg Warner ( K3 or KKK group ) from an Audi TT
This should produce up to 225 HP easily. Also a set of Ford Racing injectors, billet fuel pressure regulator, turbo tubing kit , Bosch fuel pump and GSXR 600 throttlebodies.
Read more: K04 Turbo